This might fall in the semi-obvious category, but I'd never thought to try it ... if you've got a machine with multiple CD burners that iTunes recognizes, you can burn multiple CDs at a time. Set up the playlists you'd like to burn on each drive, insert blank CDs into the drives, and start the first burn. Then go to iTunes Preferences » Advanced » Burning, and select your second (or third, etc.) burner from the CD Burner pop-up menu, click OK, and start your next playlist burning.
I burned two CDs at once on my Mac Pro, and both worked fine, so it seems iTunes can handle at least two at once. I don't have any other burners to attach, though, so I can't test more than that.
Theoretically, you might be able to AppleScript this using GUI scripting, but that's beyond my skill set.
